1. Field of the Invention
The present invention relates to a recording apparatus for forming an image on a transfer medium held on a transfer medium holding member by using a plurality of developing units.
2. Related Background Art
FIG. 19 is a cross-sectional view illustrating developing/transfer arrangement in a conventional color copying machine, and more particularly, showing a structure in which developing units are arranged around a drum.
The color copying machine includes a paper pickup roller 61 for picking up each transfer sheet 63, register rollers 62 for matching paper feeding with the image formation process and causing the transfer sheet 63 to wind around a gripper (not shown) as part of a transfer drum 64, developing units 65 to 68 arranged around a photosensitive drum 72 to sequentially develop color developing agents ( i. e. , magenta, cyan, yellow, and black), and a charger 69 for uniformly charging the photosensitive drum 72.
The color copying machine also includes a transfer charger 70 for transferring a toner image developed on the photosensitive drum 72 to the transfer sheet 63, a cleaner unit 71 for recovering residual toner particles from the photosensitive drum 72, a fixing roller 73 for fixing the toner image on the transfer sheet 63 by means of heat and pressure.
A conventional color copying machine in which the developing units are arranged around the photosensitive drum includes the transfer drum 64 obtained by winding a high-resistance film (transfer film) on a drum frame, the photosensitive drum 72, and the plurality of developing units 65 to 68 arranged around the photosensitive drum 72 so as to be selectively brought into contact therewith. The transfer sheet 63 fed by a paper feed mechanism is wound around the transfer drum 64 by the gripper arranged at part of the transfer drum 64. An image is exposed on the photosensitive drum 72. One of the plurality of developing units is brought into contact with the photosensitive drum 72 to perform the first developing cycle, and an image of the first color is transferred to the sheet at a transfer position. This operation is repeated a plurality of times to transfer toners of different colors onto the transfer sheet 63. The transfer sheet 63 is then separated from the transfer drum 64 by a separating means. The multicolor toner image is fixed by the fixing roller 73, thereby obtaining a multicolor image output.
In the conventional color copying machine, since the plurality of developing units 65 to 68 are arranged around the photosensitive drum 72 although it is difficult to obtain the photosensitive drum 72 having a uniform photosensitive film, the size of the photosensitive body, i.e., the photosensitive drum 72 is inevitably increased, resulting in high cost.
Since the size of the apparatus is increased and developing positions of the respective colors are different from each other, it is difficult to set optimal developing and transfer conditions. That is, a time period during which an image (an arrow in FIG. 19) exposed on the photosensitive drum 72 uniformly charged with the charger 69 reaches the developing unit 65 is different from time periods during which the remaining color images reach the corresponding developing units 66 to 68, and potentials in the developing timings of the respective color toners are different from each other due to dark attenuation. As a result, it is difficult to set optimal developing conditions.
In addition, since a transfer medium such as the transfer sheet 63 is wound around the transfer drum by using the gripper, only one transfer sheet 63 located at the gripper is wound around the transfer drum 64 although the transfer drum 64 has an area capable of receiving two transfer sheets 63, thus posing various problems such as a failure of a high-speed operation and a low throughput.